The cellular communication function is not only used in voice communication and data communication between cellular phones and smartphones, but also widely used in devices for M2M communication (also called MTC devices). The number of cellular communication terminals (hereinafter referred to as UEs (User Equipment) or communication terminals) including MTC devices will continue to increase in the future.
When a user and an application (hereinafter also called an MTC application, an application server, or an AS (Application Server)) operated by the user transmit application data (hereinafter also called an application PDU, an application payload, or data) to a UE, the data can be passed to an SCS (called also a Service Capability Server or an MTC server, or simply a server) to request the SCS to transmit the data to the UE (Indirect Model). The SCS that received the data from the AS uses an appropriate transmission method to transmit the data to the UE. As the transmission method, for example, there is a method of creating an IP packet addressed to the IP address of the UE to transmit the data to the UE as an application data packet, or a method of transmitting the data by including the data in a control message to be transmitted to an entity within a cellular network. In the case of using the control message, the SCS transmits the control message including the data to an IWF (Interworking Function) as a gateway to the cellular network.
This message is called a device trigger request (hereinafter also called a trigger request). The MTC-IWF that received the device trigger request selects a method of reporting trigger information such as data to the UE to start transmission processing. For a message (device trigger, which is also called a trigger below) for reporting the trigger information to the UE, there can be used a method of transmitting it as an SMS (Short Message Service) message through an SMS-SC, a method of transmitting it as a CBS (Cell Broadcast Service) message through a CBC, a method of transmitting it as an MBMS (Multicast Broadcast Message Service) message, or further a method of transmitting it as a control message to be transmitted through an entity within a core network. For example, when selecting use of the control message, the IWF transmits a control message (T5 interface) including the trigger information to an MME/SGSN/MSC, and the MME/SGSN/MSC that received the control message transmits a NAS (Network Access Stratum) message including the trigger information to the UE. The UE that received the trigger transmits a trigger response to the MME/SGSN/MSC. The application data of the UE can be included in the trigger response as necessary.
Further, when a connection for the UE to perform data communication with the SCS (called a user plane, a PDP context, or a PDN connection) is not available yet, the UE performs appropriate procedures (attachment, connection establishment, connection activation, bearer establishment, bearer activation, bearer modification) as necessary after completion of processing for the device trigger. Each of these procedures is started by transmitting a control message such as a connection establishment request, a service request, or a bearer establishment request. The UE that received the trigger establishes a connection so that the UE can acquire an IP address and transmit, to the SCS, an IP packet including the application data to be passed to the AS. On the other hand, the SCS is required to pass, to an appropriate AS, data included in the IP packet received from the UE.